When it is possible to generate a plurality of access requests for accessing a single memory, the conventional practice is to accomplish this by multiple accessing upon assigning a fixed order of priority to the sources that generate the individual access requests.
One disadvantage with the prior art is that a source generating an access request having a high order of priority continously monopolizes the memory, while the access request from a source having a low order of priority is hardly ever accepted. As a result, this source is made to wait an extremely long time for access to the memory.
Conventionally, a memory can be addressed in two ways. One method is to address the memory by means of a two dimensional address map, and the other is to employ a linear address such as an ordinary memory address. In the prior art, a CPU ordinarily is capable of addressing a memory using only one of these accessing methods.
Image data, by way of example, can be processed more efficiently by accessing an image memory using the two-dimensional addressing method. However, if it desired to use a blank space in the image memory as a work area for the CPU, efficiency suffers since the address space of the CPU is a linear address.
Conversely, if a memory address is arranged in the form of a linear address to deal with the CPU address space, the efficiency of memory utilization is improved as seen from the CPU. However, if it is desired to address the memory as e.g. an image memory by means of a matrix, the efficiency of memory utilization declines.
In order to record a full-color image with an apparatus of the kind shown in FIG. 2, four-color data for the four colors Y, M, C and BL (black) is required. Furthermore, since the recording position for each color is different, it is required that the items of color data be supplied to the recording sections for these colors at different timings. Accordingly, an arrangement has been contemplated in which each item of color data is stored in a memory and these items of data are read out of the memory at an arbitrary timing.
However, in order to store the color data representing one sheet of a full-color image, one page of memory capacity is needed for each of the colors Y, M, C and BL, so that the image memory must have a total storage capacity of four pages. This raises a problem in terms of cost. To solve this problem, consideration has been given to an arrangement in which color data is stored upon being compressed in order to reduce the required memory capacity.
If the apparatus shown in FIG. 2 is so arranged that the images of the respective colors are recorded in separate time frames, as by recording the image data for the M color after recording of the image data for the Y color is completed, then the spacing between mutually adjacent recording positions for different colors must at least be made greater than the length of the recording paper. This would result in an apparatus of very large size. Accordingly, it has been contemplated to design an apparatus of compact size by making the spacing between mutually adjacent recording positions for different colors smaller than the length of the recording paper and recording an image on the recording paper at a succeeding recording position before the same recording paper has entirely left the immediately preceding recording position.
However, when it is attempted to reduce the size of the apparatus in this manner, a plurality of image memories corresponding to the various colors are required for storing the four-color image data compressed as mentioned above. These items of color data must be read out simultaneously and decoded, and different color data must be supplied to at least two recording sections simultaneously. Though the four-color image data are indeed compressed, a total of four pages of image memory must be provided, and it is necessary to also provide a plurality of memory control circuits capable of accessing these plural image memories.